Summary At approximately 1607 Eastern standard time (EST), Air Canada Flight 457 (ACA457), an AirbusA321-211 aircraft (registrationC-GJVX, serial number1726) was on approach to Toronto/LesterB. Pearson International Airport (LBPIA), Ontario, with 123passengers and 6crew on board. At approximately 140feet above ground level (agl), on final approach to Runway24R with full flaps selected, the aircraft experienced roll oscillations. The flight crew levelled the wings and the aircraft touched down firmly. During the approach, the aircraft had accumulated mixed ice on areas of the wing and the leading edge of the horizontal stabilizer that are not protected by anti-ice systems. Approximately three hours later on the same day, Air Canada Flight1130 (ACA1130), an AirbusA321-211 aircraft (registration C-GIUF, serial number1638), with 165passengers and 7crew on board was on approach to Runway24R at LBPIA. At 1859 EST and approximately 50feet agl, the aircraft experienced roll oscillations. The flight crew conducted a go-around, changed flap settings, and returned for an uneventful approach and landing. At the gate, it was noted that the aircraft had accumulated ice on areas of the wing and the leading edge of the horizontal stabilizer that are not protected by anti-ice systems. There was no damage to either aircraft nor injury to the crew or passengers. Given the similarities between these two occurrences, this report presents the Board's investigations of both occurrences: A02O0405 (C-GJVX) and A02O0406 (C-GIUF). Ce rapport est galement disponible en franais. Other Factual Information General Both aircraft were dispatched in accordance with Air Canada's dispatch system, which included up-to-date meteorological information issued by NAV CANADA and Environment Canada. En route, the pilots received updated weather information through the datalink systems; consequently, the pilots of both aircraft were aware that moderate icing conditions were forecast for the region during the period the aircraft were scheduled to arrive at Toronto/LesterB. Pearson International Airport (LBPIA), Ontario. As the aircraft were being vectored for landing, to inch of ice accumulated on the visual ice indicators. Both aircraft used engine and wing anti-ice while flying in the icing conditions. In addition, as both aircraft were landing with full flaps (CONFIGFULL), their approach speeds (Vapp) were increased, in accordance with Air Canada's A321Aircraft Operating Manual, to VLS (lowest selectable airspeed) plus five knots to accommodate an approach conducted with CONFIGFULL in icing conditions. The autopilot systems were selected OFF between 500and 1000feet above ground level (agl) in preparation for landing. The two aircraft performed as expected until the roll oscillations began at approximately 140feet agl during the approach of ACA457 and at approximately 50feet agl for ACA1130. At that point the pilot at the control of each aircraft attempted to bring the wings level by initiating side-to-side stick movement up to the stops at a frequency of approximately 0.5hertz. Although the pilots attempted to dampen these oscillations by applying right and left stick inputs up to the stops, the magnitude of the oscillations actually increased. At this point, the pilot flying the first aircraft (ACA457) decreased power, and the aircraft touched down. In the second aircraft (ACA1130), the pilot flying decided to advance the throttles and conduct a go-around. Both decisions resulted in successful landings. In preparation for their second landing attempt, the pilots of ACA1130 decided to use CONFIG3 and adjusted the approach speed accordingly. The second approach and landing were uneventful. No other landing anomalies were reported at LBPIA during this time frame. Weather It was noted during post-landing observations by Air Canada personnel that the aircraft had accumulated as much as inch of mixed ice on those areas of the wing and horizontal stabilizer not equipped with wing anti-ice. For both flights these observations did not include information specific to ice accretion on the flap leading edge. Weather reports for the periods covering both flights forecasted icing in the region of LBPIA at altitudes between 2000feet and 9000feet above sea level (asl). Cloud top temperature was estimated to be -10C. An outside air temperature of -10C was recorded at 6000feet asl. Pilot reports (PIREPs) of moderate icing were received, the first approximately three minutes after ACA457 had landed and the second approximately one hour later. Cloud tops were reported at 5000and 7000feet asl, respectively. The PIREPs received approximately 30to 40minutes after the second occurrence indicated moderate-to-severe icing in cloud. The moderate icing reports were close to Toronto, with the cloud tops at 6000feetasl. Drizzle was reported at LBPIA at 2000feet by an observer at the surface, with the temperature at 0C, approximately 45minutes after ACA1130 landed. A surface-based precipitation occurrence sensor system at LBPIA also noted drizzle between 1929 and 2015,1 approximately 15minutes after ACA1130 landed. No drizzle was observed during the ACA457 occurrence earlier in the day. Drizzle droplet size ranged from 100to 500microns. Federal Aviation Regulation (FAR)25.1419, AppendixC envelope for certification of flight in icing conditions has maximum mean effective drop diameter between 40and 50microns. The 1600 LBPIA METAR2 was winds 220true (T) at 13knots, visibility 5statute miles (sm) in haze, ceiling 2200feet agl overcast and temperature 0C. The 1900LBPIA METAR was winds 230T at 13knots, visibility 6sm in haze, ceiling 1800feet agl overcast and temperature 0C. Roll Oscillations Flight data recorder (FDR) data relative to the occurrences were available for both aircraft. In each occurrence, the roll oscillations began at approximately 140agl for ACA457 and at approximately 50feet agl for ACA1130. Aileron inputs were applied to counter the roll oscillations with side-to-side lateral sidestick movement at a frequency of about 0.5hertz, and aileron and spoiler response appeared consistent with roll commands. There were no registered faults with the yaw damper or with any other flight control systems. The occurrence aircraft were equipped with the elevator aileron computer (ELAC) L81software. Airbus Testing Photo1. Ice accretion on main flaps leading edges from a December 1998 A321 event Modifications to the A321ELAC software were made after a landing occurrence in February2001 in which there was wing-tip damage. This occurrence also involved roll oscillations in a significant crosswind but with no icing. Consequently, revision L82to the ELAC software was introduced to reduce the roll sensitivity while in CONFIGFULL. L82modifications concern the normal, lateral flight control laws. Specifically, roll and yaw damper orders were modified to reduce roll rate and bank angle associated with rudder pedal input in CONFIG FULL below 150knots. In addition, in CONFIG3 and in CONFIG FULL in response to a roll sidestick input turn coordination, rudder activity, and lateral load factor are optimized and in response to a lateral gust induced roll effect is minimized. The L82software modification was certified and installed in four Air France A321aircraft in April2003 for a six-month in-service evaluation. The in-service evaluation of the Air France aircraft showed a reduction in roll control activity. Following the Air Canada A321 occurrences of 07December2002, Airbus analysed the available FDR data. Engineering simulations were carried out using an A321aerodynamic model to match theoretical behaviour with the actual behaviour of the aircraft on approach as observed in the ACA1130 FDR data. The simulations, however, did not take into account the actual winds nor the effects of icing on the aerodynamic response, and the model did not accurately simulate ground effect. In addition, poor sampling rates of some of the FDR parameters affected the accuracy of the simulations. Taking into account these limitations, the results suggested that there was an increase in spoiler roll efficiency during the ACA1130 occurrence compared with the theoretical model, possibly due to ice on the flaps, the effects of which were not aerodynamically modelled. The fact that ACA1130 did not encounter the roll oscillations during its second approach could be partially explained by the fact that, in CONFIG FULL, any sidestick roll demand produces immediate spoiler extension, thereby increasing roll response, whereas in CONFIG3, the spoilers will extend only if the roll order exceeds a threshold. As a result of the Air Canada occurrences, Airbus issued a Flight Operations Telex in December2002 that recommended that CONFIG3 be used for landing in conditions of anticipated moderate-to-severe icing. The Flight Operations Telex also recommended that flight in icing conditions be minimized with flaps extended, and, in the case of significant ice accumulation, the approach speed should be no lower than VLS plus10knots. To determine the effects of significant ice accretion when flaps are extended, Airbus conducted flight tests in natural icing conditions using both the A321and the A320for comparison purposes. The aircraft were flown into icing conditions with flaps extended in CONFIG2, as was the case during the initial descent of ACA1130and ACA457. Test points were conducted to assess both the clean and iced aircraft response using both roll direct and normal, lateral flight control laws.3 The testing indicated that similar amounts of ice accreted on both the non-de-iced slat and flap leading edges on both aircraft types. It was found that, in the roll direct mode, on both aircraft similar ice accretion on flaps induces a similar spoiler roll efficiency increase in both CONFIG3 and CONFIGFULL, but with the effects more pronounced in CONFIGFULL. Furthermore, ice trials performed in the normal, lateral, flight control law mode, have shown that in these conditions, roll sensitivity was increased on the A321but not ontheA320. In January2003, another roll oscillation event involving another operator occurred in crosswind landing conditions, similar to the February2001event. Further analysis by Airbus of the normal, lateral flight control laws indicated that even without ice, the A321has less stability margin than the A320, in both CONFIG3 and CONFIGFULL. The icing trials showed that the reduction in stability margin was more significant in the A321when iced up, compared to the A320under similar conditions. It was concluded that the roll sensitivity and stability margin are affected by the aerodynamic response, the normal, lateral flight control law, and resultant pilot input. To recover adequate stability margins on the non-iced A321comparable with the non-iced A320and to improve roll response in icing conditions, Airbus designed new A321normal, lateral flight control laws, which were introduced in2004. As part of this effort, wind tunnel tests were performed in August2003 using different ice shapes based on the icing trials. The goal was to develop an aerodynamic model incorporating icing effects on flaps. Following the icing tests, a further review of the crosswind events indicated that roll difficulties could be encountered under significant crosswind or moderate to severe turbulence when landing in CONFIGFULL, depending on factors such as rudder pedal inputs and gusts. The new software standards, ELAC L83and L91, consist of modifications to the normal, lateral flight control laws, which will incorporate changes based on both the crosswind and icing analyses. Airworthiness DirectiveF-2004-147, issued by France's Direction Gnrale de l'Aviation Civile (DGAC) on 18August2004 and adopted as is by Transport Canada (TC), mandates the installation of ELACL83 or L91software in A321aircraft before 31December2005. Airbus A321 Certification The degree of TC's involvement in the Airbus A321 certification was governed by the international bilateral Agreement Between the Government of Canada and the French Republic on Airworthiness. This agreement requires that the importing authority accept findings of compliance made by the exporting authority as if it had made them itself. However, the bilateral agreement also allows the importing authority to become familiar with the product and the certification process applied by the exporting authority. Using a risk management philosophy, TC determines the extent of any such familiarization on a case-by-case basis. Certification items determined to warrant more detailed familiarization are addressed by a visit to the factory. For the A321,TC did conduct a familiarization site visit to France, but the team did not include a member of the flight test group, who would normally conduct a review of performance and handling in icing. The Airbus A321-211 was certified by TC as a derivative of the Airbus A320 aircraft certification. The basis of certification for the A320is defined in TC's Type Certificate Data SheetA-166, AppendixX1. This document details the exporting authority's certification requirements and additional Canadian requirements. That same document also defines the basis of certification for the A321and details the requirements related to icing certification as Joint Aviation Requirement (JAR)25.1419 (Ice Protection) at Change11 (identical to Federal Aviation Regulation [FAR]25.1419at amendment number25-23). For the A320, to demonstrate compliance with these regulations, the exporting authority accepted the methods described in Advisory Circular-Joint ACJ1419 (Ice Protection - Interpretive Material and Acceptable Means of Compliance) in conjunction with AppendixC of FAR/JAR25. Transport Canada found that this advisory material did not adequately address the evaluation of performance and handling qualities in icing conditions. Therefore, TC imposed the additional Canadian requirement of complying with TC's Airworthiness Manual Advisory525/2 (Flight in Icing Conditions - Performance) and Airworthiness Manual Advisory525/5 (Flight in Icing Conditions - Flight Characteristics). Compliance was shown on the A320using a combination of flight tests in natural icing and in artificial ice shapes. However, the effect of ice accretion on the leading edge of the flaps was not examined during the A320certification. At the time of the A321certification, the exporting authority (DGAC) required that Airbus use the Joint Aviation Authorities' Acceptable Means of Compliance documentAMC-F14 (Flight in Icing Conditions, derived from JAANPA25F-219 at issue2, dated 22January1992) as a means of demonstrating compliance. In part, this document suggested that the certification programme address flight in icing conditions during landing. Compliance was shown on the A321using a combination of artificial ice shapes on the A321and data from A320flight tests in natural icing. Since this document adequately addressed performance and handling qualities, TCaccepted it instead of the two Airworthiness Manual Advisories it had required earlier fortheA320. Airplane-pilot Coupling Airplane-pilot coupling (APC) is commonly referred to as pilot-induced oscillations and describes a situation in which the aircraft's response is approximately 180degrees out of phase with the pilot's control input. Airplane-pilot coupling occurs when the dynamics of the aircraft, including the flight control system, and the dynamics of the pilot combine to produce an unstable aircraft. Research indicates that APC usually occurs when a pilot is engaged in a highly demanding task combined with an unforeseen external influence that causes a disturbance requiring the pilot to engage in an aggressive recovery. Typically, such disturbances happen when the aircraft is subjected to strong winds in the approach, or when airframe or controls system changes result in non-linearities in the aerodynamics of the aircraft. FAR25.143(a) and(b)4 require that an aircraft be safely controllable and manoeuvrable, without exceptional piloting skill and without danger of exceeding the aircraft limiting load factor, under any probable operating conditions. The Air Canada Flight Crew Training Manual does not contain any information regarding APCs, nor does Air Canada provide training in APC, nor is it required by regulation.